The present invention relates to a method and control for the continuous fan airflow delivered to each of several zones in a multi-zone HVAC system, and more particularly to a control that allows a specified continuous fan airflow to be delivered simultaneously to all zones of the system.
Conventional multi-zone HVAC systems include temperature changing components for changing the temperature and condition of air. Indoor air handlers drive air from the temperature changing component through supply ducts to several zones within a building. Each supply duct typically includes dampers that selectively restrict airflow into each zone to achieve a desired temperature.
Conventional systems have traditionally offered a continuous fan function that provides selection between fan “on” or fan “auto” operation. Operating the fan continuously is beneficial because air circulation equalizes temperatures and improves air quality by allowing system filters and other air quality devices to continuously clean the air. However, due to energy and noise concerns, some users may be reluctant to operate the system continuously by maintaining the system at fan “on”.
A multi-zoned HVAC system provides additional challenges regarding continuous fan operation. For air circulation, energy, noise, and size reasons, each zone may require different continuous fan speeds. Some zoned HVAC systems recognize the fan “on” setting at the thermostat for each zone and open the dampers to those zones while running the system fan at a fixed speed. Disadvantageously the fan airflow delivered to each zone is not adjustable, and the airflow delivered to each zone is dependant on that delivered to other zones. For example, in a system with two zones, with duct segments for both zones being identical and with both zones having set fan “on”, each zone will receive half of the system airflow. However, if only zone 1 has fan “on”, then the damper for zone 2 will close. Zone 1 will then receive the full system airflow, which would result in twice the amount of airflow as before. With multiple zones having varying duct sizes, the variations in continuous fan airflow delivered to any zone may be quite large which may result in unbalanced temperature changes.
Accordingly, it is desirable to provide a multi-zone HVAC system which delivers a continuous fan airflow to each zone independently of the continuous fan airflow delivered to any other zone and irrespective of the relative size difference of the zones.